1. Field of the Invention
The present invention relates generally to a Michelson interferometer used in a Fourier-transform spectrometer for detecting a component of a gas from, for example, a radiation/absorption spectrum of a gas, and more particularly to an angle detecting apparatus for detecting the angle of inclination of a scanning mirror, which is moved along an optical axis thereof with respect to the optical axis.
2. Description of the Related Art
A Fourier-transform spectrometer using a Michelson interferometer has been known as an apparatus, mounted on a spacecraft such as an artificial satellite, for observing a spectrum of infrared radiation, etc. generated from the surface of the earth and the atmosphere. In the Michelson interferometer, it is necessary to situate a fixed mirror and a scanning mirror in a fixed positional relationship at a predetermined angle defined between both mirrors, as is well known. However, the angle of the scanning mirror, as compared to the fixed mirror the position of which is stationary, is easily variable with respect to the optical axis of the scanning mirror since the scanning mirror is moved for scanning along the optical axis. In consideration of this problem, a Michelson interferometer of the type in which retro-reflectors represented by corner cubes are adopted as a fixed mirror and a scanning mirror.
However, when the Michelson interferometer is used in a dark space such as the outer space, and delicately varying infrared radiation, etc. is detected, the caliber of the corner cube must be greatly increased to obtain a necessary quantity of light.
This problem is serious in the field of space development in which limitations to weight are very strict.